gem5  v21.1.0.2
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gem5::branch_prediction::BPredUnit Class Referenceabstract

Basically a wrapper class to hold both the branch predictor and the BTB. More...

#include <bpred_unit.hh>

Inheritance diagram for gem5::branch_prediction::BPredUnit:
gem5::SimObject gem5::EventManager gem5::Serializable gem5::Drainable gem5::statistics::Group gem5::Named gem5::branch_prediction::BiModeBP gem5::branch_prediction::LocalBP gem5::branch_prediction::MultiperspectivePerceptron gem5::branch_prediction::TAGE gem5::branch_prediction::TournamentBP gem5::branch_prediction::MultiperspectivePerceptron64KB gem5::branch_prediction::MultiperspectivePerceptron8KB gem5::branch_prediction::MultiperspectivePerceptronTAGE gem5::branch_prediction::LTAGE gem5::branch_prediction::MultiperspectivePerceptronTAGE64KB gem5::branch_prediction::MultiperspectivePerceptronTAGE8KB gem5::branch_prediction::TAGE_SC_L gem5::branch_prediction::TAGE_SC_L_64KB gem5::branch_prediction::TAGE_SC_L_8KB

Classes

struct  BPredUnitStats
 
struct  PredictorHistory
 

Public Types

typedef BranchPredictorParams Params
 
- Public Types inherited from gem5::SimObject
typedef SimObjectParams Params
 

Public Member Functions

 BPredUnit (const Params &p)
 
void regProbePoints () override
 Register probe points for this object. More...
 
void drainSanityCheck () const
 Perform sanity checks after a drain. More...
 
bool predict (const StaticInstPtr &inst, const InstSeqNum &seqNum, TheISA::PCState &pc, ThreadID tid)
 Predicts whether or not the instruction is a taken branch, and the target of the branch if it is taken. More...
 
virtual void uncondBranch (ThreadID tid, Addr pc, void *&bp_history)=0
 
void update (const InstSeqNum &done_sn, ThreadID tid)
 Tells the branch predictor to commit any updates until the given sequence number. More...
 
void squash (const InstSeqNum &squashed_sn, ThreadID tid)
 Squashes all outstanding updates until a given sequence number. More...
 
void squash (const InstSeqNum &squashed_sn, const TheISA::PCState &corr_target, bool actually_taken, ThreadID tid)
 Squashes all outstanding updates until a given sequence number, and corrects that sn's update with the proper address and taken/not taken. More...
 
virtual void squash (ThreadID tid, void *bp_history)=0
 
virtual bool lookup (ThreadID tid, Addr instPC, void *&bp_history)=0
 Looks up a given PC in the BP to see if it is taken or not taken. More...
 
virtual void btbUpdate (ThreadID tid, Addr instPC, void *&bp_history)=0
 If a branch is not taken, because the BTB address is invalid or missing, this function sets the appropriate counter in the global and local predictors to not taken. More...
 
bool BTBValid (Addr instPC)
 Looks up a given PC in the BTB to see if a matching entry exists. More...
 
TheISA::PCState BTBLookup (Addr instPC)
 Looks up a given PC in the BTB to get the predicted target. More...
 
virtual void update (ThreadID tid, Addr instPC, bool taken, void *bp_history, bool squashed, const StaticInstPtr &inst, Addr corrTarget)=0
 Updates the BP with taken/not taken information. More...
 
void BTBUpdate (Addr instPC, const TheISA::PCState &target)
 Updates the BTB with the target of a branch. More...
 
void dump ()
 
- Public Member Functions inherited from gem5::SimObject
const Paramsparams () const
 
 SimObject (const Params &p)
 
virtual ~SimObject ()
 
virtual void init ()
 init() is called after all C++ SimObjects have been created and all ports are connected. More...
 
virtual void loadState (CheckpointIn &cp)
 loadState() is called on each SimObject when restoring from a checkpoint. More...
 
virtual void initState ()
 initState() is called on each SimObject when not restoring from a checkpoint. More...
 
virtual void regProbeListeners ()
 Register probe listeners for this object. More...
 
ProbeManagergetProbeManager ()
 Get the probe manager for this object. More...
 
virtual PortgetPort (const std::string &if_name, PortID idx=InvalidPortID)
 Get a port with a given name and index. More...
 
virtual void startup ()
 startup() is the final initialization call before simulation. More...
 
DrainState drain () override
 Provide a default implementation of the drain interface for objects that don't need draining. More...
 
virtual void memWriteback ()
 Write back dirty buffers to memory using functional writes. More...
 
virtual void memInvalidate ()
 Invalidate the contents of memory buffers. More...
 
void serialize (CheckpointOut &cp) const override
 Serialize an object. More...
 
void unserialize (CheckpointIn &cp) override
 Unserialize an object. More...
 
- Public Member Functions inherited from gem5::EventManager
EventQueueeventQueue () const
 
void schedule (Event &event, Tick when)
 
void deschedule (Event &event)
 
void reschedule (Event &event, Tick when, bool always=false)
 
void schedule (Event *event, Tick when)
 
void deschedule (Event *event)
 
void reschedule (Event *event, Tick when, bool always=false)
 
void wakeupEventQueue (Tick when=(Tick) -1)
 This function is not needed by the usual gem5 event loop but may be necessary in derived EventQueues which host gem5 on other schedulers. More...
 
void setCurTick (Tick newVal)
 
 EventManager (EventManager &em)
 Event manger manages events in the event queue. More...
 
 EventManager (EventManager *em)
 
 EventManager (EventQueue *eq)
 
- Public Member Functions inherited from gem5::Serializable
 Serializable ()
 
virtual ~Serializable ()
 
void serializeSection (CheckpointOut &cp, const char *name) const
 Serialize an object into a new section. More...
 
void serializeSection (CheckpointOut &cp, const std::string &name) const
 
void unserializeSection (CheckpointIn &cp, const char *name)
 Unserialize an a child object. More...
 
void unserializeSection (CheckpointIn &cp, const std::string &name)
 
- Public Member Functions inherited from gem5::Drainable
DrainState drainState () const
 Return the current drain state of an object. More...
 
virtual void notifyFork ()
 Notify a child process of a fork. More...
 
- Public Member Functions inherited from gem5::statistics::Group
 Group (Group *parent, const char *name=nullptr)
 Construct a new statistics group. More...
 
virtual ~Group ()
 
virtual void regStats ()
 Callback to set stat parameters. More...
 
virtual void resetStats ()
 Callback to reset stats. More...
 
virtual void preDumpStats ()
 Callback before stats are dumped. More...
 
void addStat (statistics::Info *info)
 Register a stat with this group. More...
 
const std::map< std::string, Group * > & getStatGroups () const
 Get all child groups associated with this object. More...
 
const std::vector< Info * > & getStats () const
 Get all stats associated with this object. More...
 
void addStatGroup (const char *name, Group *block)
 Add a stat block as a child of this block. More...
 
const InforesolveStat (std::string name) const
 Resolve a stat by its name within this group. More...
 
void mergeStatGroup (Group *block)
 Merge the contents (stats & children) of a block to this block. More...
 
 Group ()=delete
 
 Group (const Group &)=delete
 
Groupoperator= (const Group &)=delete
 
- Public Member Functions inherited from gem5::Named
 Named (const std::string &name_)
 
virtual ~Named ()=default
 
virtual std::string name () const
 

Protected Attributes

const unsigned instShiftAmt
 Number of bits to shift instructions by for predictor addresses. More...
 
- Protected Attributes inherited from gem5::SimObject
const SimObjectParams & _params
 Cached copy of the object parameters. More...
 
- Protected Attributes inherited from gem5::EventManager
EventQueueeventq
 A pointer to this object's event queue. More...
 

Private Types

typedef std::deque< PredictorHistoryHistory
 

Private Attributes

const unsigned numThreads
 Number of the threads for which the branch history is maintained. More...
 
std::vector< HistorypredHist
 The per-thread predictor history. More...
 
DefaultBTB BTB
 The BTB. More...
 
std::vector< ReturnAddrStackRAS
 The per-thread return address stack. More...
 
IndirectPredictoriPred
 The indirect target predictor. More...
 
gem5::branch_prediction::BPredUnit::BPredUnitStats stats
 

PMU Probe points.

probing::PMUUPtr ppBranches
 Branches seen by the branch predictor. More...
 
probing::PMUUPtr ppMisses
 Miss-predicted branches. More...
 
probing::PMUUPtr pmuProbePoint (const char *name)
 Helper method to instantiate probe points belonging to this object. More...
 

Additional Inherited Members

- Static Public Member Functions inherited from gem5::SimObject
static void serializeAll (const std::string &cpt_dir)
 Create a checkpoint by serializing all SimObjects in the system. More...
 
static SimObjectfind (const char *name)
 Find the SimObject with the given name and return a pointer to it. More...
 
static void setSimObjectResolver (SimObjectResolver *resolver)
 There is a single object name resolver, and it is only set when simulation is restoring from checkpoints. More...
 
static SimObjectResolvergetSimObjectResolver ()
 There is a single object name resolver, and it is only set when simulation is restoring from checkpoints. More...
 
- Static Public Member Functions inherited from gem5::Serializable
static const std::string & currentSection ()
 Gets the fully-qualified name of the active section. More...
 
static void generateCheckpointOut (const std::string &cpt_dir, std::ofstream &outstream)
 Generate a checkpoint file so that the serialization can be routed to it. More...
 
- Protected Member Functions inherited from gem5::Drainable
 Drainable ()
 
virtual ~Drainable ()
 
virtual void drainResume ()
 Resume execution after a successful drain. More...
 
void signalDrainDone () const
 Signal that an object is drained. More...
 

Detailed Description

Basically a wrapper class to hold both the branch predictor and the BTB.

Definition at line 68 of file bpred_unit.hh.

Member Typedef Documentation

◆ History

Definition at line 265 of file bpred_unit.hh.

◆ Params

typedef BranchPredictorParams gem5::branch_prediction::BPredUnit::Params

Definition at line 71 of file bpred_unit.hh.

Constructor & Destructor Documentation

◆ BPredUnit()

gem5::branch_prediction::BPredUnit::BPredUnit ( const Params p)
Parameters
paramsThe params object, that has the size of the BP and BTB.

Definition at line 59 of file bpred_unit.cc.

References gem5::SimObject::params(), gem5::MipsISA::r, and RAS.

Member Function Documentation

◆ BTBLookup()

TheISA::PCState gem5::branch_prediction::BPredUnit::BTBLookup ( Addr  instPC)
inline

Looks up a given PC in the BTB to get the predicted target.

Parameters
inst_PCThe PC to look up.
Returns
The address of the target of the branch.

Definition at line 163 of file bpred_unit.hh.

References BTB, and gem5::branch_prediction::DefaultBTB::lookup().

◆ BTBUpdate()

void gem5::branch_prediction::BPredUnit::BTBUpdate ( Addr  instPC,
const TheISA::PCState &  target 
)
inline

Updates the BTB with the target of a branch.

Parameters
inst_PCThe branch's PC that will be updated.
target_PCThe branch's target that will be added to the BTB.

Definition at line 187 of file bpred_unit.hh.

References BTB, and gem5::branch_prediction::DefaultBTB::update().

◆ btbUpdate()

virtual void gem5::branch_prediction::BPredUnit::btbUpdate ( ThreadID  tid,
Addr  instPC,
void *&  bp_history 
)
pure virtual

If a branch is not taken, because the BTB address is invalid or missing, this function sets the appropriate counter in the global and local predictors to not taken.

Parameters
inst_PCThe PC to look up the local predictor.
bp_historyPointer that will be set to an object that has the branch predictor state associated with the lookup.

Implemented in gem5::branch_prediction::MultiperspectivePerceptron, gem5::branch_prediction::TAGE, gem5::branch_prediction::TournamentBP, gem5::branch_prediction::LocalBP, and gem5::branch_prediction::BiModeBP.

Referenced by predict().

◆ BTBValid()

bool gem5::branch_prediction::BPredUnit::BTBValid ( Addr  instPC)
inline

Looks up a given PC in the BTB to see if a matching entry exists.

Parameters
inst_PCThe PC to look up.
Returns
Whether the BTB contains the given PC.

Definition at line 155 of file bpred_unit.hh.

References BTB, and gem5::branch_prediction::DefaultBTB::valid().

◆ drainSanityCheck()

void gem5::branch_prediction::BPredUnit::drainSanityCheck ( ) const

Perform sanity checks after a drain.

Definition at line 121 of file bpred_unit.cc.

References predHist.

Referenced by gem5::o3::Fetch::drainSanityCheck().

◆ dump()

void gem5::branch_prediction::BPredUnit::dump ( )

Definition at line 504 of file bpred_unit.cc.

References gem5::cprintf(), gem5::ArmISA::i, and predHist.

◆ lookup()

virtual bool gem5::branch_prediction::BPredUnit::lookup ( ThreadID  tid,
Addr  instPC,
void *&  bp_history 
)
pure virtual

Looks up a given PC in the BP to see if it is taken or not taken.

Parameters
inst_PCThe PC to look up.
bp_historyPointer that will be set to an object that has the branch predictor state associated with the lookup.
Returns
Whether the branch is taken or not taken.

Implemented in gem5::branch_prediction::MultiperspectivePerceptron, gem5::branch_prediction::MultiperspectivePerceptronTAGE, gem5::branch_prediction::TAGE, gem5::branch_prediction::LocalBP, gem5::branch_prediction::TournamentBP, and gem5::branch_prediction::BiModeBP.

Referenced by predict().

◆ pmuProbePoint()

probing::PMUUPtr gem5::branch_prediction::BPredUnit::pmuProbePoint ( const char *  name)
protected

Helper method to instantiate probe points belonging to this object.

Parameters
nameName of the probe point.
Returns
A unique_ptr to the new probe point.

Definition at line 105 of file bpred_unit.cc.

References gem5::SimObject::getProbeManager(), and gem5::Named::name().

Referenced by regProbePoints().

◆ predict()

bool gem5::branch_prediction::BPredUnit::predict ( const StaticInstPtr inst,
const InstSeqNum seqNum,
TheISA::PCState &  pc,
ThreadID  tid 
)

Predicts whether or not the instruction is a taken branch, and the target of the branch if it is taken.

Parameters
instThe branch instruction.
PCThe predicted PC is passed back through this parameter.
tidThe thread id.
Returns
Returns if the branch is taken or not.

Definition at line 130 of file bpred_unit.cc.

References gem5::StaticInst::advancePC(), BTB, gem5::branch_prediction::BPredUnit::BPredUnitStats::BTBHits, gem5::branch_prediction::BPredUnit::BPredUnitStats::BTBLookups, btbUpdate(), gem5::StaticInst::buildRetPC(), gem5::branch_prediction::BPredUnit::BPredUnitStats::condPredicted, DPRINTF, gem5::branch_prediction::IndirectPredictor::genIndirectInfo(), gem5::branch_prediction::BPredUnit::BPredUnitStats::indirectHits, gem5::branch_prediction::BPredUnit::BPredUnitStats::indirectLookups, gem5::branch_prediction::BPredUnit::BPredUnitStats::indirectMisses, iPred, gem5::StaticInst::isCall(), gem5::StaticInst::isDirectCtrl(), gem5::StaticInst::isReturn(), gem5::StaticInst::isUncondCtrl(), gem5::branch_prediction::IndirectPredictor::lookup(), gem5::branch_prediction::DefaultBTB::lookup(), lookup(), gem5::branch_prediction::BPredUnit::BPredUnitStats::lookups, gem5::MipsISA::pc, ppBranches, predHist, gem5::branch_prediction::BPredUnit::PredictorHistory::predTaken, gem5::branch_prediction::BPredUnit::PredictorHistory::pushedRAS, RAS, gem5::branch_prediction::BPredUnit::PredictorHistory::RASIndex, gem5::branch_prediction::BPredUnit::PredictorHistory::RASTarget, gem5::branch_prediction::BPredUnit::BPredUnitStats::RASUsed, gem5::branch_prediction::IndirectPredictor::recordIndirect(), stats, gem5::branch_prediction::BPredUnit::PredictorHistory::target, uncondBranch(), gem5::branch_prediction::IndirectPredictor::updateDirectionInfo(), gem5::branch_prediction::BPredUnit::PredictorHistory::usedRAS, gem5::branch_prediction::DefaultBTB::valid(), gem5::branch_prediction::BPredUnit::PredictorHistory::wasCall, gem5::branch_prediction::BPredUnit::PredictorHistory::wasIndirect, and gem5::branch_prediction::BPredUnit::PredictorHistory::wasReturn.

Referenced by gem5::o3::Fetch::lookupAndUpdateNextPC(), gem5::minor::Fetch2::predictBranch(), and gem5::BaseSimpleCPU::preExecute().

◆ regProbePoints()

void gem5::branch_prediction::BPredUnit::regProbePoints ( )
overridevirtual

Register probe points for this object.

No probe points by default, so do nothing in base.

Reimplemented from gem5::SimObject.

Definition at line 114 of file bpred_unit.cc.

References pmuProbePoint(), ppBranches, and ppMisses.

◆ squash() [1/3]

void gem5::branch_prediction::BPredUnit::squash ( const InstSeqNum squashed_sn,
const TheISA::PCState &  corr_target,
bool  actually_taken,
ThreadID  tid 
)

Squashes all outstanding updates until a given sequence number, and corrects that sn's update with the proper address and taken/not taken.

Parameters
squashed_snThe sequence number to squash any younger updates up until.
corr_targetThe correct branch target.
actually_takenThe correct branch direction.
tidThe thread id.

Definition at line 373 of file bpred_unit.cc.

References BTB, gem5::branch_prediction::IndirectPredictor::changeDirectionPrediction(), gem5::branch_prediction::BPredUnit::BPredUnitStats::condIncorrect, DPRINTF, gem5::branch_prediction::BPredUnit::BPredUnitStats::indirectMispredicted, iPred, ppMisses, predHist, RAS, gem5::branch_prediction::BPredUnit::BPredUnitStats::RASIncorrect, gem5::branch_prediction::IndirectPredictor::recordTarget(), squash(), stats, gem5::branch_prediction::DefaultBTB::update(), and update().

◆ squash() [2/3]

void gem5::branch_prediction::BPredUnit::squash ( const InstSeqNum squashed_sn,
ThreadID  tid 
)

Squashes all outstanding updates until a given sequence number.

Parameters
squashed_snThe sequence number to squash any younger updates up until.
tidThe thread id.

Definition at line 328 of file bpred_unit.cc.

References gem5::branch_prediction::IndirectPredictor::deleteIndirectInfo(), DPRINTF, iPred, predHist, RAS, and gem5::branch_prediction::IndirectPredictor::squash().

Referenced by gem5::BaseSimpleCPU::advancePC(), gem5::o3::Fetch::checkSignalsAndUpdate(), squash(), and gem5::minor::Fetch2::updateBranchPrediction().

◆ squash() [3/3]

virtual void gem5::branch_prediction::BPredUnit::squash ( ThreadID  tid,
void *  bp_history 
)
pure virtual

◆ uncondBranch()

virtual void gem5::branch_prediction::BPredUnit::uncondBranch ( ThreadID  tid,
Addr  pc,
void *&  bp_history 
)
pure virtual

◆ update() [1/2]

void gem5::branch_prediction::BPredUnit::update ( const InstSeqNum done_sn,
ThreadID  tid 
)

Tells the branch predictor to commit any updates until the given sequence number.

Parameters
done_snThe sequence number to commit any older updates up until.
tidThe thread id.

Definition at line 305 of file bpred_unit.cc.

References gem5::branch_prediction::IndirectPredictor::commit(), DPRINTF, iPred, gem5::MipsISA::pc, and predHist.

Referenced by gem5::BaseSimpleCPU::advancePC(), gem5::o3::Fetch::checkSignalsAndUpdate(), squash(), and gem5::minor::Fetch2::updateBranchPrediction().

◆ update() [2/2]

virtual void gem5::branch_prediction::BPredUnit::update ( ThreadID  tid,
Addr  instPC,
bool  taken,
void *  bp_history,
bool  squashed,
const StaticInstPtr inst,
Addr  corrTarget 
)
pure virtual

Updates the BP with taken/not taken information.

Parameters
inst_PCThe branch's PC that will be updated.
takenWhether the branch was taken or not taken.
bp_historyPointer to the branch predictor state that is associated with the branch lookup that is being updated.
squashedSet to true when this function is called during a squash operation.
instStatic instruction information
corrTargetThe resolved target of the branch (only needed for squashed branches)
Todo:
Make this update flexible enough to handle a global predictor.

Implemented in gem5::branch_prediction::MultiperspectivePerceptron, gem5::branch_prediction::MultiperspectivePerceptronTAGE, gem5::branch_prediction::TAGE_SC_L, gem5::branch_prediction::TAGE, gem5::branch_prediction::LTAGE, gem5::branch_prediction::TournamentBP, gem5::branch_prediction::LocalBP, and gem5::branch_prediction::BiModeBP.

Member Data Documentation

◆ BTB

DefaultBTB gem5::branch_prediction::BPredUnit::BTB
private

The BTB.

Definition at line 279 of file bpred_unit.hh.

Referenced by BTBLookup(), BTBUpdate(), BTBValid(), predict(), and squash().

◆ instShiftAmt

const unsigned gem5::branch_prediction::BPredUnit::instShiftAmt
protected

◆ iPred

IndirectPredictor* gem5::branch_prediction::BPredUnit::iPred
private

The indirect target predictor.

Definition at line 285 of file bpred_unit.hh.

Referenced by predict(), squash(), and update().

◆ numThreads

const unsigned gem5::branch_prediction::BPredUnit::numThreads
private

Number of the threads for which the branch history is maintained.

Definition at line 268 of file bpred_unit.hh.

◆ ppBranches

probing::PMUUPtr gem5::branch_prediction::BPredUnit::ppBranches
protected

Branches seen by the branch predictor.

Note
This counter includes speculative branches.

Definition at line 342 of file bpred_unit.hh.

Referenced by predict(), and regProbePoints().

◆ ppMisses

probing::PMUUPtr gem5::branch_prediction::BPredUnit::ppMisses
protected

Miss-predicted branches.

Definition at line 345 of file bpred_unit.hh.

Referenced by regProbePoints(), and squash().

◆ predHist

std::vector<History> gem5::branch_prediction::BPredUnit::predHist
private

The per-thread predictor history.

This is used to update the predictor as instructions are committed, or restore it to the proper state after a squash.

Definition at line 276 of file bpred_unit.hh.

Referenced by drainSanityCheck(), dump(), predict(), squash(), and update().

◆ RAS

std::vector<ReturnAddrStack> gem5::branch_prediction::BPredUnit::RAS
private

The per-thread return address stack.

Definition at line 282 of file bpred_unit.hh.

Referenced by BPredUnit(), predict(), and squash().

◆ stats

gem5::branch_prediction::BPredUnit::BPredUnitStats gem5::branch_prediction::BPredUnit::stats
private

Referenced by predict(), and squash().


The documentation for this class was generated from the following files:

Generated on Tue Sep 21 2021 12:28:46 for gem5 by doxygen 1.8.17